MgCl.sub.2.6H.sub.2 O undergoes a phase transition at about 117.degree. C. (about 243.degree. F.) releasing or alternately absorbing a substantial quantity of latent heat of fusion. As such, the material has advantageous properties for use as an encapsulated heat storage medium for use in conjunction with thermal energy storage systems, e.g., for absorption air conditioning and off peak electrical power heat storage applications. In order to improve the material for such applications, however, there is a need to develop additives or a series of additives to modify the supercooling properties of hydrated MgCl.sub.2 compositions, and most advantageously, additives which are highly effective in a very small amount.